Turboprop engines were first tested on modified B-47s. These aircraft were used to test the effectiveness of turboprops for bombers. This project was unsuccessful as well, but provided invaluable data on high-speed performance turboprop engines.
The XB-55 program developed out of the Boeing B-47 program, with a goal of increasing the cruising and top speeds and the operating altitude of the bomber. Despite the estimated potential of the B-47, the Bombardment Branch almost immediately began considering a replacement for it, stating, "the B-47.will be an exceptionally useful weapon until such time as a replacement can be made in this field."
Only two months before the XB-47's first flight, the Bombardment Branch started the XB-55 program as a replacement for the B-47 medium bomber, as well as the B-29s and B-50s still in service. Design requirements were submitted to industry in October 1947 for an airplane with a 2,000-mile radius, a 10,000-pound bomb load, and a gross weight less than 200,000 pounds.
In October of 1947 the program was officially announced with plans to develop a medium bomber. Essentially the XB-55 was larger than the B-47, using turboprop engines rather than jet engines. The new engine design, using four Allison T40-A2 turbine engines was estimated to give the XB-55 a top speed of 490 mph, but the reality was that the B-47 still had far better performance ratings.
The turboprop uses a gas turbine core to turn a propeller. Propellers develop thrust by moving a large mass of air through a small change in velocity. Propellers are very efficient and can use nearly any kind of engine to turn the prop. There are two main parts to a turboprop propulsion system, the core engine and the propeller. The core is very similar to a basic turbojet having a compressor, burner, and turbine. However, at the exit of the main turbine the hot exhaust gas is passed through an additional turbinec, before entering the nozzle. Unlike a basic turbojet, most of the energy of the exhaust is used to turn this additional turbine. The turbine is attached to an additional drive shaft which passes through the core shaft and is connected to a gear box. The gear box is then connected to a propeller that produces most of the thrust. The exhaust velocity of the core is low and contributes little thrust because most of the energy of the core exhaust has gone into turning the drive shaft.
On 01 July 1948 Boeing received the contract to develop the XB-55. The airplane submitted by Boeing was based around the Allison T40 turboprop engine. Allison, apart from proceeding toward an improved version of the J35 (later designated J71), also attempted turboprop designs, notably the T38 and T40. From that experience came the very successful T56. In those early days of engine development, new engines were essentially larger versions, incorporating up-rated technology, of engines already in production. Most new engine programs started from a base of development money added to a production engine contract.
The unusual Allison T40 turboprop engine combined two T38 gas turbine power sections that drove a common gearbox (turboprop engines typically have only one gas turbine power section). A unique version, the XT40-A-1, powered the experimental XF-84H aircraft. While most T40 engines drove contra-rotating propellers, the XF-84ís engine drove a single propeller shaft. Allison built only two XT40-A-1 engines, and the T40 series was not built in large quantity due to delays and reliability problems.
The original requirement for all-around defensive armament was relaxed to allow for nose and tail armament only, a factor that reduced the weight of the airplane to allow it to meet the range requirement.
Over the next few years, the XB-55 program evolved into a paper study on a turbojet configuration, and investigations were even conducted on a delta-wing configuration. The XB-55 did not withstand budget crunches of the late 1940s and was cancelled in January 1949). By this time, production of the B-47B, with accommodations to carry a nuclear bomb, and begun, and the Air Force instead decided to stay with the B-47B as its medium bomber.
Because propellers become less efficient as the speed of the aircraft increases, today turboprops are used only for low speed aircraft, like cargo planes. High speed aircraft usually use high bypass turbofans because of the high fuel efficiency and high speed capability of turbofans. A variation of the turboprop engine is the turboshaft engine. In a turboshaft engine, the gear box is not connected to a propeller but to some other drive device. Turboshaft engines are used in many helicopters, as well as tanks, boats, and even race cars in the late 1960's.
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